1. Field of the Invention
The present invention relates to a less expensive method for manufacturing a display device, which makes use of techniques of partial release and transfer to a substrate.
2. Description of the Related Art
Time division gradation drive is a digital drive method for displaying intermediate gradations by using electroluminescence (hereinafter referred to as EL) elements. This time division gradation drive method has been disclosed, for example, in the publication “Active Organic electroluminescence Displays” (2001 FPD Technology Outlook, pp. 747–762), Japanese Laid-Open Patent Applications 2001-166730 and 2001-166749, and elsewhere. This time division gradation drive method provides high image quality with a relatively simple structure.
The above publication discloses a circuit featuring one thin film transistor (hereinafter referred to as a TFT) as an example of a circuit when the electro-optical elements are liquid crystals in a pixel circuit for driving the electro-optical element provided to each pixel in a time division gradation drive method. Japanese Laid-Open Patent Application 2001-166730 discloses a circuit featuring three TFTs as an example of a circuit when electro-optical elements are used as EL elements. An advantage of a display device employing time division gradation drive is the simplicity of the circuit structure, which means that fewer TFTs or other elements are needed.
In the past, there was no good way to manufacture a display device when only a few circuit elements were used, as in a time division gradation drive method. In order to form the display area of a display device, pixel circuits including TFTs were usually manufactured by employing a semiconductor process, and while only a tiny area was needed to form pixel circuits with respect to the surface area involved in image display, this semiconductor process had to be applied to the entire substrate. Consequently, much of the material used in manufacture was wasted, which was one factor in higher manufacturing cost.
Also, the characteristics of EL elements may be compromised by the high temperatures necessary to form TFTs, and this imposed limitations on the manufacturing procedure in that the EL elements could not be formed until after the TFTs had been manufactured first. This problem can be solved by using inter-substrate transfer technology in which a TFT layer formed on a first substrate is released and transferred to a second substrate to manufacture a device, as has been discussed in detail in Japanese Laid-Open Patent Applications H10-125929, H10-125930, and H10-125931 under the title “Release Method.” Even with this technology, though, since TFTs are only used in a portion of the wider display area, the release and transfer must be performed for a large-surface area substrate, which wastes material and leads to higher manufacturing cost.
In light of the above problems, it is an object of the present invention to provide a method for manufacturing a display device at a lower cost through a reduction in the number of elements used in the pixel circuits.